Wave climate variability in the North Atlantic in recent decades in the winter period using numerical modeling

The study focuses on investigating significant wave height, including both mean and extreme values, in the North Atlantic in winter during the period from 1979 to 2010. We perform a 32-year wind wave hindcast for the North Atlantic using a spectral ocean wave model (WaveWatch III) and a high-resolution nonhydrostatic atmospheric model (WRF-ARW), which provides the wind forcing function. Analysis of the 32-year hindcast of wave characteristics in the North Atlantic reveals stronger mean and extreme waves simulated with high resolution modeling systems and identifies significant downward trends in the mean significant wave height in the subpolar North Atlantic. Such trends were not found in the wave characteristics from ERA-Interim reanalysis. At the same time, the 32-year hindcast did not confirm the statistically significance of strong positive trends in the central Atlantic diagnosed by ERA-Interim reanalysis; differences between the reanalysis and hindcast are discussed.     

Estimating the reproduction quality of precipitation over the north atlantic and influence of the hydrostatic approximation in the WRF–ARW atmospheric model

The Weather Research and Forecast numerical model (WRF) with the dynamic Advanced Research WRF (ARW) solver was used to simulate the winter (January 2016) and summer (July 2015) atmospheric state over the North Atlantic with a high (15 km) spatial resolution. The quality of precipitation modeling was validated by remote sensing Global Precipitation Measurements (GPM) data and atmospheric ERA-Interim reanalysis. Nonhydrostatic and hydrostatic equations for the vertical velocity were additionally used to investigate their influence on the accuracy of the precipitation modeling results. It was shown that the model in this configuration satisfactorily reproduces the precipitation field. No evidence of hydrostatic approximation was revealed (over a simulation domain with a resolution of 15 km, simplified topography, and parameterizations of convection and microphysical processes).     

Climate teleconnections at monthly time scales in the Ligurian Sea inferred from satellite data

The existence and spatial distribution of possible teleconnections between the South Pacific and North Atlantic oceans and the Ligurian Sea (North-western Mediterranean) are investigated in the present paper. Teleconnections are searched by cross-correlating monthly spatio-temporal time series of 1.1 km resolution sea surface temperature (SST), and a 22.2 km resolution sea level anomaly (SLA), measured from satellite from March 1993 to August 1999, with two indices characterising the South Pacific and the North Atlantic variability: the Southern Oscillation (SO) and the North Atlantic Oscillation (NAO) indices, respectively. Concerning the variability induced by the North Atlantic Ocean, it is shown that it mostly influences the SLA field in the Ligurian Sea. Specifically, relevant anti-correlations between SLA and North Atlantic variability have been found in all the Ligurian sub-basin. As expected by geographical proximity, the effects of North Atlantic on the SLA field in the Ligurian Sea are instantaneous at monthly time scales. Instead, correlations between SST and NAO Index are found at time lag τ = 1 month in the southern part of the basin highlighting the memory of the ocean related to their heat capacity. Significant anti-correlations between SO Index and the SST field in the Ligurian Sea, were obtained at time lag τ = 4 months in the coastal areas of the sub-basin. Results also indicate that the impact of teleconnections in the area studied is not geographically uniform.     

Pacific tropical–extratropical thermocline water mass exchanges in the NCAR Coupled Climate System Model v.3

In this study we document how model biases in extratropical surface wind and precipitation, due to ocean–atmosphere coupling, are communicated to the equatorial Pacific thermocline through Pacific Subtropical Cell (STC) pathways. We compare the simulation of climate mean Pacific Subtropical Cells (STCs) in the NCAR Community Climate System Model version 3 (CCSM3) to observations and to an uncoupled ocean simulation (the ocean component of the CCSM3 forced by observed wind stress and surface fluxes). We use two versions of the CCSM3 with atmospheric resolution of 2.8° (T42) and 1.4° (T85) to investigate whether the climate mean STCs are sensitive to the resolution of the atmospheric model.Since STCs provide water that maintains the equatorial thermocline, we first document biases in equatorial temperature and salinity fields. We then investigate to what extent these biases are due to the simulation of extratropical–tropical water mass exchanges in the coupled models. We demonstrate that the coupled models’ cold and fresh bias in the equatorial thermocline is due to the subduction of significantly fresher and colder water in the South Pacific. This freshening is due to too much precipitation in the South Pacific Convergence Zone. Lagrangian trajectories of water that flows to the equatorial thermocline are calculated to demonstrate that the anomalously large potential vorticity barriers in the coupled simulations in both the North and South Pacific prevent water in the lower thermocline from reaching the equator. The equatorial thermocline is shown to be primarily maintained by water that subducts in the subtropical South Pacific in both the coupled and uncoupled simulations. It is shown that the zonally integrated transport convergence at the equator in the subsurface branch of the climate mean STCs is well simulated in the uncoupled ocean model. However, coupling reduces the net equatorward pycnocline transport by 4 Sv at 9°S and 1 Sv at 9°N. An increase in the atmospheric resolution from T42 to T85 results in more realistic equatorial trades and off-equatorial convergence zones.     

Dissolved organic carbon, nitrogen and phosphorus in the N-E Atlantic and the N-W Mediterranean with particular reference to non-refractory fractions and degradation

Dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) measured in deep profiles in the N-E Atlantic and in the N-W Mediterranean in the period 1984–2002 are described. After accurate validation, they show close agreement with those previously published.Classic profiles were obtained, with concentrations decreasing in deep waters. In the Mediterranean and in the Atlantic comparable concentrations were found in the 1500–2000 m waters, 44–46 μmol l⁻¹ DOC, 2.6–2.8 μmol l⁻¹ DON and 0.02–0.03 μmol l⁻¹ DOP. In the surface layers, DOC concentrations were higher, but DON and DOP concentrations lower, in the Mediterranean than in the Atlantic, leading to higher element ratios in the Mediterranean. In autumn, values were, respectively, DOC:DON 17 vs. 14, DOC:DOP 950 vs. 500 and DON:DOP 55 vs. 35. The data suggest an increase in DOC and DON in the North Atlantic Central Water over 15 years, which may be linked to the North Atlantic climatic oscillations.Refractory DOM found in the 1500–2000 m layer exhibited C:N:P ratios of 1570:100:1. The labile+semi-labile (=non-refractory) DOM (nrDOM) pool was computed as DOM in excess of the refractory pool. Its contribution to total DOM above the thermocline in the open sea amounted to 25–35% of DOC, 30–35% of DON, and 60–80% of DOP. Element ratios of the nrDOM varied among stations and were lower than those of refractory DOM, except for C:N in the Mediterranean: nrDOC:nrDON 10–19, nrDOC:nrDOP 160–530 and nrDON:nrDOP 15–38. The specific stoichiometry of DOM in the Mediterranean led us to postulate that overconsumption of carbon is probably a main process in that oligotrophic sea.By coupling non-refractory DOM stoichiometry and relationships between the main DOM elements in the water column, the relative mineralization of C, N and P from DOM was studied. Below the thermocline, the preferential removal of phosphorus with regard to carbon from the semi-labile DOM can be confirmed, but not the preferential removal of nitrogen. In the ocean surface layers, processes depend on the oceanic area and can differ from deep waters, so preferential carbon removal seems more frequent. Bacterial growth efficiency data indicate that bacteria are directly responsible for mineralization of a high proportion of DON and DOP in the deep water.     

Hydrographic Vertical Separation Surfaces (HyVSEPs) for the Tidal Waters of Canada

Since the advent of Global Navigation Satellite Systems, it has been possible to perform hydrographic survey reductions through the ellipsoid, which has the potential to simplify operations and improve bathymetric products. This technique requires a spatially continuous separation surface connecting chart datum (CD) to a geodetic ellipsoid. The Canadian Hydrographic Service (CHS), with support from the Canadian Geodetic Survey, has developed a new suite of such surfaces, termed Hydrographic Vertical Separations Surfaces, or HyVSEPs, for CD and seven tidal levels. They capture the spatial variability of the tidal datum and levels between tide gauges and offshore using semiempirical models coupling observations at tide stations with relative sea-level rise estimates, dynamic ocean model solutions, satellite altimetry, and a geoid model. HyVSEPs are available for all tidal waters of Canada, covering over seven million square kilometers of ocean and more than 200,000 kilometers of shoreline. This document provides an overview of the CHS's modeling approach, tools, methods, and procedures.

The HyVSEP for CD defines the new hydrographic datum for the tidal waters of Canada. HyVSEPs for other tidal levels are fundamental for coastal studies, climate change adaptation and the definition of the Canadian shoreline and offshore boundaries. HyVSEPs for inland waters are not discussed.     

A Coupled Ice-Ocean Model in the Pan-Arctic and North Atlantic Ocean: Simulation of Seasonal Cycles

A coupled ice-ocean model is configured for the pan-Arctic and northern North Atlantic Ocean with a 27.5 km resolution. The model is driven by the daily atmospheric climatology averaged from the 40-year NCEP reanalysis (1958–1997). The ocean model is the Princeton Ocean Model (POM), while the sea ice model is based on a full thermodynamical and dynamical model with plastic-viscous rheology. A sea ice model with multiple categories of thickness is utilized. A systematic model-data comparison was conducted. This model reasonably reproduces seasonal cycles of both the sea ice and the ocean. Climatological sea ice areas derived from historical data are used to validate the ice model performance. The simulated sea ice cover reaches a maximum of 14 × 10⁶ km² in winter and a minimum of 6.7 × 10⁶ km² in summer. This is close to the 95-year climatology with a maximum of 13.3 × 10⁶ km² in winter and a minimum of 7 × 10⁶ km² in summer. The simulated general circulation in the Arctic Ocean, the GIN (Greenland, Iceland, and Norwegian) seas, and northern North Atlantic Ocean are qualitatively consistent with historical mapping. It is found that the low winter salinity or freshwater in the Canada Basin tends to converge due to the strong anticyclonic atmospheric circulation that drives the anticyclonic ocean surface current, while low summer salinity or freshwater tends to spread inside the Arctic and exports out of the Arctic due to the relaxing wind field. It is also found that the warm, saline Atlantic Water has little seasonal variation, based on both simulation and observations. Seasonal cycles of temperature and salinity at several representative locations reveals regional features that characterize different water mass properties.     

Assessment of excess nitrate development in the subtropical North Atlantic

Geochemical estimates of N₂ fixation in the North Atlantic often serve as a foundation for estimating global marine diazotrophy. Yet despite being well-studied, estimations of nitrogen fixation rates in this basin vary widely. Here we investigate the variability in published estimates of excess nitrogen accumulation rates in the main thermocline of the subtropical North Atlantic, testing the assumptions and choices made in the analyses. Employing one of these previously described methods, modified here with improved estimates of excess N spatial gradients and ventilation rates of the main thermocline, we determine a total excess N accumulation rate of 7.8 ± 1.7 × 10¹¹ mol N yr^− 1. Contributions to excess N development include atmospheric deposition of high N:P nutrients (adding excess N at a rate of 3.0 ± 0.9 × 10¹¹ mol N yr^− 1 for  38% of the total), high N:P dissolved organic matter advected into and mineralized in the main thermocline (adding excess N at 2.2 ± 1.1 × 10¹¹ mol N yr^− 1 for  28% of the total), and, calculated by mass balance of the excess N field, N₂ fixation (adding excess N at 2.6 ± 2.2 × 10¹¹ mol N yr^− 1 for  33% of the total). Assuming an N:P of 40 and this rate of excess N accumulation due to the process, N₂ fixation in the North Atlantic subtropical gyre is estimated at  4 × 10¹¹ mol N yr^− 1. This relatively low rate of N₂ fixation suggests that i) the rate of N₂ fixation in the North Atlantic is greatly overestimated in some previous analyses, ii) the main thermocline is not the primary repository of N fixed by diazotrophs, and/or iii) the N:P ratio of exported diazotrophic organic matter is much lower than generally assumed. It is this last possibility, and our uncertainty in the N:P ratios of exported material supporting excess N development, that greatly lessens our confidence in geochemical measures of N₂ fixation.     

Modeling ocean primary production: Sensitivity to spectral resolution of attenuation and absorption of light

Modeling the vertical penetration of photosynthetically active radiation (PAR) through the ocean, and its utilization by phytoplankton, is fundamental to simulating marine primary production. The variation of attenuation and absorption of light with wavelength suggests that photosynthesis should be modeled at high spectral resolution, but this is computationally expensive. To model primary production in global 3d models, a balance between computer time and accuracy is necessary. We investigate the effects of varying the spectral resolution of the underwater light field and the photosynthetic efficiency of phytoplankton (α^*), on primary production using a 1d coupled ecosystem ocean turbulence model. The model is applied at three sites in the Atlantic Ocean (CIS (60°N), PAP (50°N) and ESTOC (30°N)) to include the effect of different meteorological forcing and parameter sets. We also investigate three different methods for modeling α^* – as a fixed constant, varying with both wavelength and chlorophyll concentration [Bricaud, A., Morel, A., Babin, M., Allali, K., Claustre, H., 1998. Variations of light absorption by suspended particles with chlorophyll a concentration in oceanic (case 1) waters. Analysis and implications for bio-optical models. J. Geophys. Res. 103, 31033–31044], and using a non-spectral parameterization [Anderson, T.R., 1993. A spectrally averaged model of light penetration and photosynthesis. Limnol. Oceanogr. 38, 1403–1419]. After selecting the appropriate ecosystem parameters for each of the three sites we vary the spectral resolution of light and α^* from 1 to 61 wavebands and study the results in conjunction with the three different α^*estimation methods. The results show modeled estimates of ocean primary productivity are highly sensitive to the degree of spectral resolution and α^*. For accurate simulations of primary production and chlorophyll distribution we recommend a spectral resolution of at least six wavebands if α^* is a function of wavelength and chlorophyll, and three wavebands if α^* is a fixed value.     

Effects of different closures for thickness diffusivity

The effects of spatial variations of the thickness diffusivity (K) appropriate to the parameterisation of [Gent, P.R. and McWilliams, J.C., 1990. Isopycnal mixing in ocean circulation models. J. Phys. Oceanogr., 20, 150–155.] are assessed in a coarse resolution global ocean general circulation model. Simulations using three closures yielding different lateral and/or vertical variations in K are compared with a simulation using a constant value. Although the effects of changing K are in general small and all simulations remain biased compared to observations, we find systematic local sensitivities of the simulated circulation on K. In particular, increasing K near the surface in the tropical ocean lifts the depth of the equatorial thermocline, the strength of the Antarctic Circumpolar Current decreases while the subpolar and subtropical gyre transports in the North Atlantic increase by increasing K locally. We also find that the lateral and vertical structure of K given by a recently proposed closure reduces the negative temperature biases in the western North Atlantic by adjusting the pathways of the Gulf Stream and the North Atlantic Current to a more realistic position.     

Deep convection in the ocean general circulation model: Variability on the diurnal,seasonal, and interannual time scales

Features of geographical localization and of temporal variability of convective mixing were examined based on numerical experiments with the general ocean circulation model developed at the Hydrometeorological Research Center of the Russian Federation. The computations were performed using 6-h data on atmospheric forcing, which allows one to simulate the variability in a broad range of time scales—from diurnal to interannual. On the whole, the results of the numerical experiments are consistent with the available scarce observational data available on the deep convection in the North Atlantic. A pronounced regionalization of the deep convection in the open ocean is noted, as well as a significant spatial and temporal intermittence of convective events on time scales from a day to a few days. On the interannual time scale, a correlation is recognized with the variations of the atmospheric forcing and with the hydrophysical conditions in the ocean, in particular, with the cyclonic circulation within the baroclinic layer.     

The effect of ocean tides on a climate model simulation

We implemented an explicit forcing of the complete lunisolar tides into an ocean model which is part of a coupled atmosphere–hydrology–ocean–sea ice model. An ensemble of experiments with this climate model shows that the model is significantly affected by the induced tidal mixing and nonlinear interactions of tides with low frequency motion. The largest changes occur in the North Atlantic where the ocean current system gets changed on large scales. In particular, the pathway of the North Atlantic Current is modified resulting in improved sea surface temperature fields compared to the non-tidal run. These modifications are accompanied by a more realistic simulation of the convection in the Labrador Sea. The modification of sea surface temperature in the North Atlantic region leads to heat flux changes of up to 50 W/m². The climate simulations indicate that an improvement of the North Atlantic Current has implications for the simulation of the Western European Climate, with amplified temperature trends between 1950 and 2000, which are closer to the observed trends.     

Atlantic multidecadal oscillation and its manifestations in the Atlantic-European region

We present a brief survey of the works devoted to the investigation of the Atlantic Multidecadal Oscillation, i.e., of the quasiperiodic variations of sea-surface temperature in the North Atlantic with typical time scales of 50–100 yr. This oscillation is a manifestation of the natural variability in the ocean-atmosphere system. The characteristic scale of the Atlantic Multidecadal Oscillation is determined by the speed of the meridional oceanic circulation in the North Atlantic. The analyzed oscillation affects various climatic characteristics: air temperature, river discharge in the European and North-American regions, the number and intensity of tropical cyclones in the Atlantic Ocean, and the parameters of mid-latitude cyclones and anticyclones in the Atlantic-European region. The main mechanism by which the Atlantic Multidecadal Oscillation affects the climatic characteristics of the regions neighboring with the North Atlantic is the atmospheric response to the thermal anomalies in the ocean leading to a shift of the centers of atmospheric action and to the changes in the intensity and predominant directions of propagation of atmospheric cyclones and anticyclones. By using the results of long-term instrumental observations carried out in Eastern Europe and the data array of reconstructed temperature in the Alpine region, it is shown that the Atlantic Multidecadal Oscillation is responsible for a significant part of low-frequency variations of temperature in Europe. This fact confirms the potential predictability of the regional atmospheric manifestations of the Atlantic Multidecadal Oscillation on the decadal-scale. Translated from Morskoi Gidrofizicheskii Zhurnal, No. 4, pp. 69–79, July–August, 2008.     

Stochastic study of the temperature response of the upper ocean to uncertainties in the atmospheric forcing in an Atlantic OGCM

The impact of errors in atmospheric forcing on the behaviour of ocean models is a fundamental issue for ocean modellers and data assimilation and one that has yet to be fully addressed. In this study, we use a stochastic modelling approach with 50 7-months (September–March) primitive equation eddy permitting (1/4°) integrations. We investigate the response of the oceanic circulation to atmospheric uncertainties, focusing principally on their impact on the upper oceanic temperature field. The ensemble is generated by perturbing the wind, atmospheric temperature and incoming solar radiation of the ERA40 reanalysis. Each perturbation consists of a random combination of the 20 dominant EOFs of the difference between the ERA40 and NCEP/CORE reanalysis datasets. The ensemble standard deviation of various interfacial and oceanic quantities is then examined in the upper 200 m of three distinct regions of the North Atlantic: in the Gulf Stream, in the Northern Tropical band and in the North East Atlantic. These show that even a very small perturbation of the atmospheric variables can lead to significant changes in the ocean properties and that regions of oceanic mesoscale activity are the most sensitive. The ocean response is driven by vertical diffusivity and eddy activity. The role of subsurface currents is also crucial in carrying the eddy signal away from the regions of mesoscale activity. Finally, the decorrelation time scale of the mesoscale activity is critical in determining the amplitude of the oceanic response.     

Simulation of the interannual and seasonal variability of the overflow transport through the Denmark Strait

The character of the water exchange in the Denmark Strait for the period of 1958–2006 is studied based on the results of the numerical experiments using the model of the ocean circulation developed at the Institute of Numerical Mathematics of the Russian Academy of Sciences with a resolution of 0.25 degrees in latitude and longitude with 27 vertical levels. The calculations were performed for the North Atlantic area from 30° S, including the Arctic Ocean and the Bering Sea. The width of the Denmark Strait at 66° N is about 650 km, and the depth is approximately 550 m. The fields of the temperature, salinity, and density and the components of the current velocities were simulated. In this period, the average overflow of dense waters with the conventional potential density σ₀ > 27.80 to the North Atlantic through the Denmark Strait was 1.86 ± 0.96 Sv, and, for the nearbottom and intermediate waters with σ₀ > 27.50, it was 3.84 ± 1.31 Sv. The maximum values of the overflow transport through the strait were recorded in 1962, 1972, 1983, 1990, and 2000. Exactly these years showed the highest values of the North Atlantic oscillation (NAO) index. This fact confirms the domination of the decadal variability of the hydrogeological processes in the North Atlantic. The model section of the current velocity through the strait showed the occurrence of at least four well marked jets that vertically occupy the entire sectional area from the surface to the bottom. The two jets divided by a northward jet at the strait’s middle move southward along the Greenland slope. The northward current along Iceland is also identified. This structure of the currents is also supported by the analysis of the observed variability of the absolute topography of the ocean’s surface.     

Impact of Ocean Mean Dynamic Topography on Satellite Data Assimilation

The response of an eddy-permitting ocean model to changes imposed by the use of different mean dynamic topographies (MDT) is analyzed in a multivariate assimilation context, allowing the evaluation of this impact, not only on the surface circulation, but also on the interior ocean representation. The assimilation scheme is a reduced-order sequential Kalman filter (SEEK). In a first set of experiments, high resolution sea surface temperature, along-track sea surface height and sea surface salinity from climatology are assimilated into a 1/3° resolution North and Tropical Atlantic version of the HYCOM model. In a second experiment, in situ profile data are assimilated in addition to the surface measurements.

The first set of experiments illustrates that important differences in the representation of the horizontal model circulation pattern are related to differences in the MDT used. The objective of assimilation is to improve the representation of the 3D ocean state. However, the imperfect representation of the mean dynamic topography appears to be an important limiting factor with regard to the degree of realism obtained in the simulated flow.

Vertical temperature and salinity profiles are key observations to drive a general circulation ocean model toward a more realistic state. The second set of experiments shows that assimilating them in addition to sea surface measurements is a far from trivial exercise. A specific difficulty is due to inconsistencies between the dynamic topography diagnosed from in situ observations and that diagnosed from sea surface height. These two fields obtained from different data sources do not contain exactly the same information. In order to overcome this difficulty, a strategy is proposed and validated.     

On the performance of a shipborne current mapping HF radar

High-frequency (HF) radars have been developed to map surface currents offshore by means of land-based stations. Presently available radar systems use frequencies between 25 and 30 MHz and allow a spatial resolution of 1 km and ranges of up to 50 km. This paper reports on the experience with a shipborne radar and discusses problems which arise for the azimuthal resolution on a metal ship, the correction for the ship's speed, and limitations due to pitch-and-roll motions. Current measurements during cruises to the North Atlantic are presented. It has been found that, with the support of the satellite-supported Global Positioning System, the shipborne HF radar can measure surface current velocities with an accuracy of some 5 cm·s^-1     

Interannual to decadal Gulf Stream variability in an eddy-resolving ocean model

Meridional shifts of the Gulf Stream (GS) jet on interannual to decadal timescales and the corresponding oceanic changes around the GS are investigated using a near global eddy-resolving ocean model hindcast from 1960 to 2003. The simulated variability in the shifts of the GS jet axis shows good agreement with observations, and lags atmospheric fluctuations characterized by the North Atlantic Oscillation by about 2 years. This lagged response of the GS jet to the atmospheric variations is attributed to the westward propagation of the undulation of the jet axis from 45°W to 75°W, which has a wavelength of about 4000 km and a displacement of 0.5°. The propagation direction and phase speed of about 2.8 cm s⁻¹ are consistent with the thin-jet theory. The shifts of the jet axis in the downstream region are likely induced by wind fluctuations through Ekman convergence over the central North Atlantic. Associated with the northward (southward) shift of the jet axis, sea surface temperature is warming (cooling) around and north of the jet, and the former warming has a deep and meridionally narrow subsurface structure, consistent with the northward shift of the jet. The meridional shifts of the jet accompany coherent meridional shifts of energetic eddy activity regions around the GS. Our numerical results suggest that the GS jet brings the atmospheric signals from the central to the western North Atlantic, and the resultant meridional shift of the jet induces the notable oceanic changes around the GS.     

Seasonal evolution of the mediterranean water propagation in the north Atlantic

The Climatic variability of the seasonal water exchange in the Strait of Gibraltar and the spatial structure of the tongue of the Mediterranean Waters (MW) in the Atlantic Ocean are studied. The analysis is based on the results of a numerical experiment using a 3D ocean circulation model developed at the Institute of Computational Mathematics (ICM RAS) with a resolution of the dataset over the latitude and longitude equal to 0.25 degree. The seasonal evolution of the salinity and density fields is calculated in the North Atlantic (from 20 °S, including the Mediterranean Sea) and in the Arctic Ocean (including the Bering Sea). The comparison of the model estimates with the results of field observations demonstrated good agreement. The transport of the MW into the Atlantic is close to the observed data (during the year, it varies from 1.8 Sv in the winter to 0.9 Sv in the summer). The complex pattern of the currents in the region of the MW spreading is confirmed. The alternating character of the saline MW tongue at the 1000-m level is shown for the first time. It is found that the zones of maximum salinities in the open part of the ocean coincide with the main trajectories of migration of MW lenses and the regions of their decomposition.     

利用气候态水文数据Hydrobase II分析北欧海深层水的分布与输运

Deep water in the Nordic seas is the major source of Atlantic deep water and its formation and transport play an important role in the heat and mass exchange between polar and the North Atlantic. A monthly hydrological climatology—Hydrobase II—is used to estimate the deep ocean circulation pattern and the deep water distribution in the Nordic seas. An improved P-vector method is applied in the geostrophic current calculation which introduces sea surface height gradient to solve the issue that a residual barotropic flow cannot be recognized by traditional method in regions where motionless level does not exist. The volume proportions, spatial distributions and seasonal variations of major water masses are examined and a comparison with other hydrological dataset is carried out. The variations and transports of deep water are investigated based on estimated circulation and water mass distributions. The seasonal variation of deep water volume in the Greenland Basin is around 22×103 km3 whereas significantly weaker in the Lofoten and Norwegian Basins. Annual downstream transports of about 1.54×103 and 0.64×103 km3 are reported between the Greenland/Lofoten and Lofoten/Norwegian Basins. The deep water transport among major basins is generally in the Greenland-Lofoten-Norwegian direction.